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Suppose two persons are to
communicate with one another.
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To communicate successfully, they
should be sharing a common language.
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Therefore, both ends should be able to
understand what the other person is saying.
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Similarly, in computer networks, the computers
should be sharing a common message format.
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They should know how long the message
is? Which part of the message is the
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actual data? Which part of the message
is the sender's and receiver's address?
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Such information will result in successful
communication between computers.
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If one computer speaks ASCII and the other
speaks Unicode, successful communication
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will not occur unless they are prepared to
perform the translations back and forth.
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So, some ground rules are required
to communicate successfully.
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In computer networks, the agreed-upon set
of ground rules that make communication
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possible are called protocols.
TCP/IP is a set of protocols that
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support network communication, but what is
a network and what is communication?
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In the most basic form, two computers
connected via LAN Cable sharing data with
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the help of Network Interface Cards (hardware
present in each computer) forms a network,
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and the process of sending messages from
one place to another through a wired or
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wireless medium is called communication.
The message can be a file, a voice conversation,
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a streaming video or anything which can
be communicated in digital form.
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These messages are not sent
as a single unit; instead,
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they are broken into small data units. These
data units are transmitted through the network
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and restored at the receiver
into the original message.
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In TCP/IP protocol suite, TCP breaks messages
into small data units called segments and hands
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them off to IP, which deals with routing segments
through the networks to their final destination.
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TCP module in the receiver combines the
segments to form the original message.
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Note that TCP stands for Transmission Control
Protocol and IP stands for Internet Protocol.
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An alternative to TCP is UDP. It stands for User
Datagram Protocol. The main difference is that TCP
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is highly reliable, but it is slow, whereas UDP
is less reliable but generally faster. Both TCP
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and UDP are a part of the protocol suite.
However, due to heavy dependence on TCP,
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and for historical reasons, the entire set
of protocols is referred to as TCP/IP.
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TCP/IP is a network model designed to support
network communication, even if the computers are
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from different manufacturers. There is one
more network model called the OSI model or
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Open System Interconnection reference model. It
is primarily used for research. On the other hand,
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TCP/IP is a practical model developed to meet
the needs of the original Internet design.
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As per the name, TCP/IP seems to be a set
of two protocols only – TCP and IP. However,
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it consists of numerous protocols
bundled at different layers.
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The topmost layer is the Application layer which
generates a message. The message is passed to the
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lower layers at the sending node, where each layer
encapsulates the message from the above layer.
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So, the message sent becomes larger and
larger as it passes down the chain.
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The data unit in the data link
layer is called an Ethernet frame;
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in the network layer, it is called an IP
packet; if it is in the transport layer,
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it is called TCP segment in case of TCP protocol,
and UDP datagram in case of UDP protocol.
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In the application layer, it is
called an application message.
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The peer layer removes the header at the
receiving node and passes the remainder
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upwards layer-by-layer till the message
finally reaches the application layer.
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Let us discuss each layer
one-by-one in more detail.
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We are starting with the bottom-most
layer – the physical layer.
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The physical layer is the place where
actual communication takes place.
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We know that a sequence of 0s and 1s
digitally represents the messages.
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The physical layer converts this binary sequence
into signals and transmits them over local media.
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The signal can be electrical if the
local media is Copper Cable or LAN cable,
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the Light signal in case of Optical Fiber
and a Radio signal in case of Air/Vacuum.
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So, the signal generated by the Physical Layer
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depends on the type of media
used to connect two devices.
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The most common protocol used at
the physical layer is Ethernet.
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The protocol also specifies the type of
cables that can be used for data transmission.
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For example, if the protocol used is Ethernet,
then twisted pair cable, coaxial cable, or fiber
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optic cable can be used for data transmission.
If the protocol used is fast Ethernet or gigabit
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ethernet, then twisted pair or fiber optic
cable can be used as local media.
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Next is - THE DATA LINK LAYER
The data unit in the data link
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layer is called an Ethernet frame. The data
link layer is divided into two sublayers:
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· Medium-access control or MAC sublayer, and
· Logical link control or LLC sub-layer
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The MAC sublayer is responsible for
· Data encapsulation, and
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· Accessing the media
In data encapsulation,
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the MAC sublayer adds a header and a trailer to
the IP packet received from the network layer.
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The header contains the MAC addresses
of the sender and receiver. The trailer
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contains 4 bytes of error checking data used to
detect errors in the received Ethernet frame.
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What is the MAC address? It is a unique
6-byte address embedded in the NIC of
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a device by its manufacturer.
For accessing the media, the access
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method Ethernet uses is called Carrier Sense
Multiple Access/Collision Detection or CSMA/CD.
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In this method, each computer listens to the
cable before sending data through the network.
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If the network is clear, the computer will
transmit. If the first computer is already
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transmitting on the cable, the second computer
will wait and try again when the line is clear.
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Sometimes, two computers attempt to transmit
at the same instant. When this happens,
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a collision occurs. Each computer then stops
transmission and waits a random amount of time
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before attempting to retransmit.
Please note, with this access method;
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it is normal to have collisions. However, the
delay caused by collisions and retransmitting
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is very small and does not normally affect
the speed of transmission on the network.
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The next is the LLC sub-layer. It offers
· flow control, and
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· error control
Flow control is a technique that restricts
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the amount of data that a sender can send without
overwhelming the receiver. The receiving devices
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have a limited processing speed and a limited
memory to store the incoming data. If these limits
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are exceeded, then the incoming data will be lost.
To avoid this, the receiver should inform the
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sender to slow down the transmission rate before
these limits are met. In the data link layer, flow
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control restricts the number of frames the sender
can send without overwhelming the receiver.
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Error control in the data link layer primarily
refers to error detection and retransmission.
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Error detection is done by using the error
checking bytes added in the trailer of the frame.
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The frame retransmission is done
using Automatic Repeat Request
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or ARQ. The receiver sends an ACK to
the sender when a frame is received.
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When the ACK is not received, the sender sends the
frame again. So, if a frame gets lost or damaged,
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then the ACK is not sent. As a result, the
sender sends the frame again. This process
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is called Automatic Repeat Request (ARQ).
LLC layer can also re-size the IP packets
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received from the network layer to fit
them in the data link layer frames.
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The transport layer provides most
of the services of the LLC sublayer,
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including flow control, error control, and
sizing of packets; therefore, the services
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of the LLC layer are usually bypassed.
The remaining three layers of the TCP/IP
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protocol stack, including network,
transport, and application layers,
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are implemented as software programs
within the computer's operating system.
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Starting with the NETWORK LAYER
The transport layer passes TCP segments
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or UDP datagrams to the Network Layer. The
network layer adds logical addresses or IP
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addresses to the TCP segments or UDP datagrams to
form IP packets and then uses routers to send the
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IP packets to other networks. The network layer
also determines the best path for data delivery.
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So the functions of the network layer are:
1. Logical Addressing
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2. Routing
3. Path determination
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IP is the single standard protocol for this
layer. The TCP/IP network layer is also
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called the internetworking layer or IP layer.
Logical Addressing: Every computer in a network
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has a unique IP address. The network layer
assigns sender and receiver's IP addresses
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to each segment or datagram to form an IP
Packet. IP addresses are assigned to ensure
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that each IP packet can reach the correct
destination present in different networks.
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Routing
Routing is
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a method of moving an IP packet from source to
destination present in different networks. Routing
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is not needed if the source and destination
computers are present in the same network.
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For communications within a
network, the task is usually simple.
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The ARP module takes the destination IP address
from the IP packet and returns the MAC address
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of the destination computer. It is then used
to create an Ethernet frame which is delivered
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directly to the destination as it is present
in the same network, . no routing is needed.
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However, when the message is being sent to a node
outside a network, for example, to the Internet,
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the network layer moves the message from sender
to receiver through routers. Consider two networks
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connected with a router. Computer A needs to
send data to computer B. Please note that both
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computers are present in different networks.
Hence, in this case, routing is needed.
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To create an Ethernet frame, we need the
MAC address of the destination computer.
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However, in this case, the destination is present
in a different network. So, the ARP module cannot
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provide us with the destination's MAC address
because it can provide the MAC address only if
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the computers are present in the same network. So,
the ARP module in network one cannot provide the
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MAC addresses of the computers present in network
two and vice-versa. Since the intermediate to the
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networks is the router R, the destination MAC
address is kept as the router's MAC address, and
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the frame is forwarded to the router. Router finds
that the MAC address in the frame matches its
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address. So, it extracts the IP packet from the
frame and forwards it to the network layer. The
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network layer finds a mismatch for the destination
IP address. So it sends the IP packet down to
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the data link layer and updates the destination
MAC address with the MAC address of computer B.
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But how router knows the MAC address of computer
B? Simple, by using the ARP module. It is one
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network, so the ARP module works here. Finally,
the ethernet frame is delivered to computer B.
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Please note that the destinations IP address
never changes for inter-network communication,
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but the physical address or the MAC
address changes with every hop.
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So, IP addresses are a must to transfer
data among multiple networks.
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Now Path determination:
A computer can be connected
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to an internet server or a computer in several
ways. Choosing the best possible path for data
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delivery from source to destination is called
Path Determination. Layer 3 devices use protocols
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such as OSPF (Open Shortest Path First), BGP
(Border Gateway Protocol), IS-IS (Intermediate
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System to Intermediate System) to determine
the best possible path for data delivery.
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Because routing takes place at the network
layer or layer 3, routers and gateways
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are sometimes called layer three switches.
IP is unreliable. It does not guarantee delivery
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nor check for errors. These tasks are the
responsibility of the transport layer.
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Let us start discussing the transport layer
At the sending node, the transport layer
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receives the message from the application layer.
When the message reaches the transport layer,
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one of the transport layer protocols,
i.e., TCP or UDP, is selected.
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TCP supports segmentation. So, if the message
is large, TCP divides it into smaller pieces
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and adds a header to form a TCP segment.
On the other hand, UDP does not support
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segmentation, so the applications
using UDP should send messages
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short enough to fit into one UDP datagram.
Note that the data unit in TCP is called
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TCP segment, and the data unit in
UDP is called UDP datagram.
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UDP datagrams are considered unreliable because
there is no guarantee that all datagrams sent
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will be received in the destination and in the
correct order. So, if reliability is needed,
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UDP should not be used.
UDP lacks error checking and
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correction. It makes UDP fast and efficient
for DNS, DHCP, SNMP, and RIP protocols.
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UDP is also suited for streaming videos.
When the application layer invokes the UDP
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protocol, UDP encapsulates the application
message into UDP datagrams. The datagram is then
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passed to the network layer for transmission.
At the receiving end, the network layer sends
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the UDP datagram to the transport layer. UDP then
extracts the application message from the datagram
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and passes it to the application layer.
TCP, on the other hand, is reliable and
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guarantees in-order delivery of data from the
sender to the receiver. The data transmission
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via TCP has three phases:
· Connection establishment
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· Data transfer, and
· Connection termination
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In the Connection Establishment phase, the sender
TCP or client sends a packet to the receiver TCP
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or server requesting a connection. The server then
sends an acknowledgement to the client. The client
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further acknowledges the server. It completes
the process of connection establishment. Since a
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connection is set up before data transmission,
TCP is a connection-oriented protocol,
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and the connection establishment process is
called Three-Way TCP Connection Handshake.
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Once the connection is established,
the next phase is the Data Transfer.
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During data transfer, TCP offers some key features
which UDP does not provide, and it includes
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• Error-free data transfer
• Ordered-data transfer
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• Retransmission of lost data
• Discarding duplicate packets, and
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• Congestion throttling
Let us discuss each feature one-by-one.
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Error-Free Data Transfer is provided by using
the field Checksum. The sender calculates and
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enters a value in this field. At the receiving
end, the receiver performs the same process
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and calculates the checksum value. If it does
not match with the value present in the checksum
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field, the TCP segment is discarded, and no
ACK is sent to the sender. Because the sending
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side does not receive an acknowledgement of
the discarded packet, it is retransmitted.
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Ordered-Data Transfer
TCP adds a sequence number in the TCP segments.
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At the receiving end, the TCP module uses the
sequence numbers to reconstruct the application
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message in the correct order.
Retransmission of Lost segments
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For reliable data transfer, the receiver TCP sends
an acknowledgement to the sender TCP for each TCP
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segment it receives. If an acknowledgement is
not received, the TCP segment is retransmitted.
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Therefore, if a TCP segment is lost, the receiver
will not send an ACK message to the sender.
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As a result, the sender TCP
sends the lost segment again.
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Discarding Duplicate segments
The TCP client retransmits packets that
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it determines to be lost. However, the receiver
TCP may receive segments that were considered to
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be lost after the sending side has retransmitted
the segments. As a result, the receiving end will
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have two or more copies of the same segment. In
such cases, the unique sequence numbers in the TCP
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header of every segment helps to determine the
duplicate segments, which are then discarded.
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Congestion Throttling or flow control
The goal for TCP is to send segments to
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the receiving end as fast as possible without
losing them. When TCP first sends the segments,
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it sets a timer. If the segments are acknowledged
before the timer expires, TCP increases the
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transmission speed until the segments begin to
become unacknowledged. Since the ACK for some
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segments is not received within the time, the
sending TCP module retransmits the segments.
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When a significant number of packets have
to be retransmitted, TCP slows down the
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data transfer rate. In this way, TCP handles
congestion throttling or flow control.
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The last phase in the data transmission
is Connection Termination.
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When an endpoint wishes to stop its connection,
it sends a finished message to the other endpoint.
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The other end acknowledges the message. Both
ends do this two-phase handshake process.
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Therefore, the connection termination follows
a four-way handshake process.
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The Top Most Layer in the
TCP/IP protocol suite is the
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Application layer
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The application layer is used by user applications
that pass messages from one computer to another in
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layer protocols to perform their activities.
For example, web browsers use HTTP
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or HTTPS – to do web surfing. Email
programs, such as Microsoft Outlook, use
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post office protocol (POP) or the Simple Mail
Transfer Protocol (SMTP) for transferring emails.
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So, the application layer provides means
to access information on the network.
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This is a list of protocols
provided by the application layer
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DNS - Domain Name System translates IP
addresses into Domain Names and vice-versa.
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DHCP - Dynamic Host Configuration Protocol
automatically assign IP addresses to
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computers present in the network.
FTP - File Transfer Protocol is used
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to transfer files on the Internet
HTTP - HyperText Transfer Protocol
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is used for sending and receiving webpages
IMAP - Internet Message Access Protocol is used
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for email messages on the Internet
IRC - Internet Relay Chat protocol
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is used for Internet chat.
POP3 - Post Office Protocol
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Version 3 is used by email clients to
retrieve messages from remote servers
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SMTP - Simple Mail Transfer Protocol is
used for email messages on the Internet
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It completes the TCP/IP protocol suite.
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